1. Field of the Invention
The invention relates to a scale which operates according to the principle of electromagnetic force compensation, with at least one coil positioned in the air gap of a stationary permanent magnet system and acted upon, via a position sensor and a control amplifier, by a compensation direct current dependent on the load of the scale, and a precision resistor for measuring purposes through which the same compensation direct current flows and at both ends of which a signal dependent on the load of the scale can be tapped off and be fed to an analog/digital converter.
2. Prior Art
Because of joulean heat in such scales, the load-dependent compensation direct current causes a load-dependent temperature increase of the coil and the measuring resistor. These temperature variations at varying load of the scale are transmitted from the coil and the measuring resistor to the other parts of the scale. The parallel construction for the load scale is especially sensitive to temperature variations. When, e.g., the parallel construction is constructed according to U.S. Pat. No. 4,062,416, the guide that is closer to the coil is heated more quickly and strongly than the other guide. This results in mechanical stresses within the parallel construction and therefore in zero point variations of the scale. The load-dependent temperature increase in the measuring resistor results in small but noticeable sensitivity variations, particularly in scales with high resolution. In the use of the scale, intermissions and periods of small loads and periods with large loads follow each other statistically, so that various average temperatures as well as various chronological and local temperature gradients appear. The zero point and sensitivity variations caused thereby limit the meaningful resolution of the scale.
Some scales are provided with an additional correction coil in the permanent magnet system which is to offset the feedback of the coil to the permanent magnet system, as is described, e.g., in U.S. Pat. No. 2,780,101, and explained as to its mode of functioning. This additional correction coil is also traversed by the load-dependent compensation direct current and presents a further load-dependent heat source which likewise contributes to the temperature variations described.
In order to diminish the disadvantages described, it has already been proposed (U.S. Pat. No. 4,134,468) to provide near the coil additional heat-generating means which is electrically connected in such a manner that the sum of the heating powers at no load and full load value is substantially the same. This procedure, however, on the one hand, does not take into account at all the variable dissipation loss in the measuring resistor, so that the error effect thereof remains fully preserved. On the other hand, with respect to the coil, there is also only a partial efficiency.
Either the additional heat-generating means is arranged outside the air gap of the permanent magnet system at a short distance from the coil, so that at varying loads a variation in the location of the heat-generation results, or the further heat-generating means is positioned as a thin layer within the air gap of the permanent magnet system. In the first arrangement, varying temperature gradients result, and in the second arrangement, the same heat generation in the coil and in further heat-generating means causes different temperature increases, and the heating and cooling time constants are also different. In this case too, therefore, temperature conditions vary with the load. In a third alternative, the further heat-generating means may be arranged in the same size as the coil (and thus with the same specific heat generation) within the air gap of the permanent magnet system. In this case, however, only one half of the air gap volume can be utilized for electromagnetic force compensation, so that for a prespecified permanent magnet system and at a prespecified bearing capacity the heat generation in the coil is twice as great and therefore the thermal errors become even more conspicuous.
Moreover, it has already been proposed (German Pat. No. 27 22,093) to pass through the coil and measuring resistor, alternately, a current in positive and negative direction, in which case the relative switching-on duration of the two directions is controlled in a load-dependent manner. By this procedure, to be sure, a load-independent heat generation in the coil and in the measuring resistor is achieved, but the circuits are expensive since two voltage sources of different polarities are necessary and since the inductivity of the coil produces at the switching of the current direction high voltage peaks.